This invention relates to an optical system based on a liquid crystal phase.
Related arrangements are known from IEEE Trans. Electron Devices ED-26 (1979) 1734 which disclosure is incorporated by reference herein. Therein is described a schlieren-optical system made as follows. Light of a relatively narrow frequency band is aligned parallel through a first lens system, then sent through a phase grid and subsequently projected onto a screen via a projection lens. The phase grid here is realized by a liquid crystal cell. Specifically, this cell contains two glass plates, the inner surface of each of which carries a fine pattern of strip electrodes parallel to one another and an orientating layer, the plates enclosing a liquid crystal layer between them. The liquid crystal has a positive dielectric anisotropy and, at rest, has a privileged direction parallel to the extension of the electrodes.
If an electrical voltage is applied to the electrodes of this cell, those liquid crystal molecules which sense an E-field are tilted in the direction of the field. This produces the result that light which oscillates parallel to the extension of the electrodes finds in the liquid crystal layer a refractive index modulated with the grid period and is correspondingly diffracted. The diffraction angle of a maximum of a particular order is given by the geometry of the arrangement and the wavelength of the light, while its intensity can be controlled by the voltage applied.
The type of cell described allows switching times of less than 15 msec, but with the precondition of an extremely thin liquid crystal layer of about 2 .mu.m, which imposes extremely high requirements on the production of the cell. If a change is made to larger distances between the plates, both the switch-on and the switch-off time increase disproportionately (compare FIG. 4 in the reference). Moreover, even values of the order of magnitude of 10 msec are still too high when large amounts of information--for example video signals--are to be processed. There is also another disadvantage: light oscillating at right angles to the extension of the electrodes senses (virtually) nothing of the change in refractive index induced by the field; it therefore does not take part in the diffraction and does not contribute to the effect. Last but not least, the condition that the liquid crystal layer must be preorientated is also unsatisfactory; the cited publication envisages vapor-deposition processes in vacuo for this. These cause considerable trouble in production of the cells.